The present invention relates to decoders for processing image data which has been compressed according to a format, MPEG-2, specified by the moving pictures experts group (MPEG) and in particular to a preprocessing step which selectively deletes unneeded User Data from the MPEG-2 data stream prior to decoding.
Video signal compression performed under the MPEG-2 standard is inherently variable rate. Video data in a sequence of images is compressed based on the spatial frequency content of either a sequence of images or of differences in spatial frequency content among the images in the sequence. If an image sequence has low spatial frequency content or if successive images differ only slightly, the amount of compressed data that is transmitted to reproduce the image sequence may be greatly reduced.
The syntax for the MPEG-2 standard is set forth in International Standard 13818-2 Recommendation ITU-T H.262 entitled xe2x80x9cGeneric Coding of Moving Pictures and Associated Audio Information: Videoxe2x80x9d and available from ISO/IEC, Geneva, Switzerland, which is incorporated herein by reference for its teaching of the MPEG-2 video coding standard. This standard defines several layers of data records which are used to convey both audio and video data. For the sake of simplicity, the decoding of the audio data is not described herein. Encoded data which describes a particular video sequence is represented in several nested layers, the Sequence layer, the Group of Pictures layer, the Picture layer, the Slice layer and the Macroblock layer. Each layer record, except for the Macroblock layer, begins with a start code that identifies the layer. The layer record includes header data and payload data.
Under the MPEG-2 standard, a prescribed amount of data is received and stored before the decoding operation may begin. This amount of data is specified in the header portion of the Sequence layer as the value vbv_buffer_size_value. This value also determines an amount of memory to be reserved for a buffer, the VBV buffer, which holds the bit-stream data before it is decoded.
As described below with reference to FIGS. 3A through 3D, many of the records of the MPEG-2 bit-stream include an Extension Data field and/or a User Data field which may comprise many bytes of data. This Extension Data is provided in the bit-stream to allow the MPEG-2 standard to be extended without redefining its basic syntax. The User Data does not represent an extension of the MPEG-2 standard. It is provided as a means for a signal provider to furnish special-purpose information to those who receive the signal. It may be used for any purpose that is agreed to by the signal provider and the decoder manufacturer. For example, it may be used to send production information from a main signal source (e.g. a network production facility) to a signal distributor (e.g. a network member station). Other uses for this data may be to send multi-lingual close-caption information or other data that may overlay the image such as, for example, a custom on-screen display.
While a particular receiver may be configured to process some of the User Data records, other User Data may be unknown to the receiver and thus, may be superfluous.
The present invention is embodied in an MPEG-2 decoder which includes a bit-stream parser that identifies and removes superfluous User Data from an MPEG-2 bit-stream before storing the bit-stream into the VBV buffer of the decoder.
According to one aspect of the invention, the parser includes circuitry which detects the level in the bit-stream syntax at which the User Data occurs and circuitry which selectively removes only the User Data which is associated with selected levels.
According to yet another aspect of the invention, the levels at which User Data is to be removed are provided to the parser by a microprocessor and may be programmed.